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Evolution of Specific Heat Capacity with Temperature for Typical processes Article Evolution of Specific Heat Capacity with Temperature for Typical Supports Used for Heterogeneous Catalysts Xiaojia Lu 1,2, Yanjun Wang 1, Lionel Estel 1 , Narendra Kumar 2, Henrik Grénman 2,* and Sébastien Leveneur 1,* 1 Normandie Univ, INSA Rouen, UNIROUEN, LSPC, EA4704, 76000 Rouen, France; [email protected] (X.L.); [email protected] (Y.W.); [email protected] (L.E.) 2 Laboratory of Industrial Chemistry and Reaction Engineering, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FI-20500 Åbo/Turku, Finland; narendra.kumar@abo.fi * Correspondence: henrik.grenman@abo.fi (H.G.); [email protected] (S.L.); Tel.: +33-2329-566-54 (S.L.) Received: 25 June 2020; Accepted: 20 July 2020; Published: 1 August 2020 Abstract: Heterogeneous catalysts are widely used in the chemical industry. Compared with homogeneous catalysts, they can be easily separated from the reaction mixture. To design and optimize an efficient and safe chemical process one needs to calculate the energy balance, implying the need for knowledge of the catalyst’s specific heat capacity. Such values are typically not reported in the literature, especially not the temperature dependence. To fill this gap in knowledge, the specific heat capacities of commonly utilized heterogeneous catalytic supports were measured at different temperatures in a Tian–Calvet calorimeter. The following materials were tested: activated carbon, aluminum oxide, amberlite IR120 (H-form), H-Beta-25, H-Beta-38, H-Y-60, H-ZSM-5-23, H-ZSM-5-280, silicon dioxide, titanium dioxide, and zeolite 13X. Polynomial expressions were successfully fitted to the experimental data. Keywords: specific heat capacity; heterogeneous catalytic material; micro-calorimeter C80 1. Introduction Catalysts play a crucial role in the modern chemical industry, and indirectly the development of society. According to statistics, there are around 30,000 different raw materials and chemical intermediates that are synthesized by using catalysts. These materials are not only related to people’s food, clothing, and housing, but also involve modern high-tech fields such as information transmission, network technology, aerospace [1–3], and bioengineering [4,5]. Today, researchers are committed to developing more efficient, selective, less expensive, and greener industrial catalysts in order to upgrade current chemical production technologies. Heterogeneous catalysts are the most widely used catalysts in industrial production, due to their versatile physicochemical properties, high hydrothermal stability, and efficient catalyst recovery/reusability [6,7]. Indeed, heterogeneous catalysis contributes to about 90% of chemical production processes and to more than 20% of all industrial products [8]. There are numerous types of catalytic materials and supports, among which zeolite, mesoporous catalyst, resin catalyst, alumina, and activated-carbon-based catalysts are typically used in industry. Usually, the characteristics of a catalyst, such as specific surface area, particle size, morphology, porosity, and acidity, are determined, as they are crucial parameters for performance and use in industrial applications. However, the effect of the heat capacity of catalysts on large-scale chemical processes has not received much attention in the literature, even though the thermodynamic properties vary significantly between different catalysts Processes 2020, 8, 911; doi:10.3390/pr8080911 www.mdpi.com/journal/processes Processes 2020, 8, 911 2 of 12 or catalyst supports. Industrial processes can be performed under non-isothermal conditions with different types of catalysts and catalyst loadings, and many processes are performed in continuous reactors with high catalyst loads. Determining the thermal properties of a catalyst is necessary when evaluating the thermal risk of a process performed under non-isothermal conditions, especially when considering larger-scale production. Besides risk assessment, the heat capacity of the catalyst can influence the operation and temperature profile of a reactor significantly. This issue often arises at the start of a process. The heat capacity of a large catalyst bed can have a significant impact on the energy balance. These aspects can be very important for robust modeling and simulation purposes, which are the basis of the reliable design and operation of chemical processes. However, much attention has typically been focused on determining the physical properties of the bulk phase [9–11] in detail, including heat capacity, while very little data are found for heterogeneous catalysts [12,13]. Differential scanning calorimetry (DSC) [14] is a conventional method for measuring a variety of thermodynamic properties. It has been widely applied to the specific heat capacity (Cp) measurement of different materials, such as frying oil [15], alloys [16], phase change materials [17], and solid lead [18]. However, the sensitivity of measuring and calculating specific heat capacity by general DSC is usually very low, resulting in a relative error of 5–10%, mainly due to the weak baseline reproducibility. In addition, poor correction accuracy is obtained by this method. The calibration of general DSC instruments is usually performed by melting the standard metal, and it is greatly affected by the sample morphology, reaction type, and atmosphere. Moreover, sample adaptability is very poor. The sample is always required to maintain good contact with the bottom of the crucible to ensure excellent heat conduction; thus, the Cp test of powders and large heterogeneous samples is greatly restricted. Furthermore, liquid samples are also not applicable, as small crucibles (<100 µL) are usually used, which are difficult to seal. The Setaram C80 3D Calvet Calorimeter is powerful and flexible. Measurement by C80 is similar to DSC, i.e., the energy difference between the measured substance and the reference substance is determined under a programmed-temperature control. However, the Calvet calorimeter possesses a caloric efficiency of up to 94%, whereas that of typical plate DSC is between 20 and 40%. Besides this, the detector adopts a three-dimensional full-clad calorimetric method, and, contrary to the general DSC, it takes into account the heat flow that may occur inside the sample itself during calorimetry. The C80 measurement is independent of the weight, form, and nature of the sample, the type of cell utilized, the manner of contact between sample and sensor, and the nature of the sweeping gas (inert, oxidizing, wet, etc.), providing an excellent precision, even for samples with irregular shapes or uneven heat conduction. Furthermore, the temperature is raised and lowered at a very consistent, slow rate, which makes the Cp measurement more accurate. The C80 calorimeter has found an application in the precise determination of heat capacities of different materials [19–23]. In this work, a micro-calorimeter (C80) was for the first time applied for determining the specific heat capacity of commonly used catalytic materials/supports as a function of temperature. These data contribute to filling an important gap in knowledge in this area. The data can be used to estimate the kinetic constants and reaction enthalpies of chemical processes at both laboratory and industrial scale. Moreover, the heat capacities are important basic data for safety and risk assessment purposes. 2. Materials and Methods 2.1. Calorimetric Reactor Rystem The C80 Tian–Calvet calorimeter (Figure1), manufactured by Setaram instrumentation, has absolute calibration, featuring a three-dimensional transducer with maximum sensitivity. Its measuring temperature range is between ambient (298 K) and 573 K. The apparatus is equipped with twin detectors and twin cells (a measurement cell and a reference cell), which are surrounded by thermocouples. External thermal interferences in the calorimetric system, such as phi-factor [24], are eliminated by a differential coupling of the measurement and reference detectors, allowing for Processes 2020, 8, x 3 of 12 determination of the heat flow for radiation, convection, and conduction in a very precise way. The standard error of the temperature measurement is 0.1 K, and the standard error of enthalpy measurement is 0.1%. The C80 calorimeter has been successfully applied previously in studying different processes, such as hydration, dehydration, denaturation, dissolution, gas adsorption, phase transition, and monomer polymerization. Moreover, the C80 is operated by Setsoft 2000 (Setaram thermal analysis software), and heat measurement can be carried out under an isothermal mode or a temperature-programmed mode, which makes it an ideal device for the measurement of Cp. The current study employed a C80 micro-calorimeter for the measurement of the specific heat capacity of different catalytic materials, in the temperature range 313–453 K. In order to determine the heat flow (energy absorbed by the samples) at different temperatures, a pair of Hastelloy reversing mixing cells was employed. In the C80, the heat flow determined is proportional to the Cp value. Hence, the heat capacity is calculated directly from the heat flow signal. The isothermal baselines before and after each temperature rise need to be long enough for the system to stabilize and reach stationary conditions. The accuracy of the instrument was successfully verified by sodium chloride before the formal test. The samples were dried overnight in an oven at 393 K before the measurement. The measurement cell was filled
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